Data from: The effect of competition on responses to drought and inter-annual climate variability of a dominant conifer tree of western North America

To accurately predict how ecosystems will respond to climate change — and how management actions can influence such responses — scientists and managers need a better understanding of how and when biotic interactions modify climate-growth relationships. However, current research has largely ignored the role of competition in modulating climate-growth relationships of mature trees. In this study, we assessed the effect of competition on tree responses to drought and inter-annual climate variability as well as linkages between climate sensitivity and morphological characteristics of the stemwood. We sampled 10 sites in northeastern Washington, USA, and used dendroecology to quantify responses of Douglas-fir (Pseudotsuga menziesii) to drought and climate variability. Tree ring series were converted to basal area increment (BAI) series and the effects of competition on climate-growth relationships were assessed at the tree and site levels using a combination of correlation analyses and linear mixed-effects models. Competition did not affect tree responses to extreme drought. When soil moisture was below average, tree growth was tightly coupled to climate variability for all trees, regardless of their competitive status. However, in wet years, competition had a pronounced, positive effect on climate sensitivity of growth. Trees with more competition from neighbors exhibited a significantly higher proportion of sapwood area in latewood (a morphological trait associated with greater drought resistance). Synthesis. Our results suggest that a tree's ability to cope with environmental variability is driven not just by the proximate effects of neighbors on resource availability, but also by phenotypic plasticity and long-term adaptations to competitive stress (such as changes in carbon allocation). Findings have important implications for individual-tree and stand-level growth models and may help managers better understand how their activities will modify tree responses to climate change.

为精准预测生态系统对气候变化的响应，以及管理措施如何影响此类响应，科研人员与管理者亟需深入理解生物交互如何、在何时调控气候-生长关系。然而，当前研究大多忽视了竞争对成熟树木气候-生长关系的调控作用。
在本研究中，我们评估了竞争对树木响应干旱与年际气候变率的效应，同时探究了气候敏感性与茎干木材形态特征之间的关联。我们在美国华盛顿州东北部选取10个样地，采用树木年代生态学（dendroecology）方法量化花旗松（Pseudotsuga menziesii）对干旱与气候变率的响应。将树木年轮序列转换为断面积增量（BAI）序列，并结合相关分析与线性混合效应模型，在单株树木与样地两个尺度上评估竞争对气候-生长关系的影响。
竞争并未显著影响树木对极端干旱的响应。当土壤湿度低于平均水平时，无论树木所处的竞争状态如何，所有个体的生长均与气候变率紧密相关。但在湿润年份，竞争对生长的气候敏感性具有显著的正向调控效应。受邻株竞争更强的树木，其晚材中的边材面积占比显著更高——这一形态特征与更强的抗旱性相关。
综合分析：本研究结果表明，树木应对环境变异的能力不仅取决于邻株对资源可获得性的直接影响，同时也受表型可塑性以及对竞争胁迫的长期适应性调控（例如碳分配模式的改变）。本研究结果对单株树木与林分尺度的生长模型具有重要参考价值，同时可帮助管理者更深入地理解其经营活动如何改变树木对气候变化的响应。